FIGS. 2(a) and 2(b) show a plan view and a cross sectional view, respectively, of a prior art photoconductivity type infrared detector.
In these figures, a compound semiconductor such as Cd.sub.x Hg.sub.1-x Te or InSb 2 is disposed on a high resistance substrate 1 comprising CdTe. A pair of electrodes 3a and 3b are disposed on the compound semiconductor 2 opposite each other. A light receiving section 4 is disposed at the surface of the compound semiconductor 2 between the electrodes 3a and 3b. Reference numeral 5 designates paths of bias currents.
The device will operate as follows.
In order to extend the wavelength sensitivity of the photoconductivity type infrared detector beyond that of infrared rays, Cd.sub.x Hg.sub.1-x Te or InSb having a narrow energy band gap is used as the compound semiconductor 2. When an infrared ray having larger energy than the energy band gap of that material is incident on the light receiving section 4, excess carriers are generated by the light that is absorbed by the compound semiconductor 2. The variation in the electrical conductivity due to these excess carriers is output as a signal in use, the detector is cooled down to about 77 K. by using liquid nitrogen to limit the dark current.
Furthermore, when the element resistance is r(.OMEGA.), the bias current i.sub.B is (A), and the voltage between the electrodes 3a and 3b is V (V), the voltage V becomes EQU V=r.multidot.i.sub.B. (1)
Since the bias current i.sub.B in the formula (1) is usually constant, the variation in the element resistance .DELTA.r due to the excess carriers is detected as a signal V.sub.s (V), that is, as a variation .DELTA.V in the voltage from the following formula EQU V.sub.s =.DELTA.V=Vr.multidot.i.sub.B. (2)
On the other hand, the element resistance r becomes ##EQU1## Herein, l, w, and t represent the length, width, and thickness of the light receiving section 4, respectively. Furthermore, n (cm.sup.-3) represent the carrier concentration of the compound semiconductor 2, and k represents a proportionality constant.
Usually the light receiving section 4 is a square shape, and then l=w. Furthermore, when the excess carriers generated in the compound semiconductor due to the incident energy .PHI..sub.S (W) are .DELTA.n (cm.sup.-3), .DELTA.n is EQU .DELTA.n=k'.multidot..PHI..sub.S, (4)
herein, k' is a proportional constant. The variation .DELTA.r in the element resistance r due to this .DELTA.n becomes as in the formula (5) from the formula (3) ##EQU2## When the sensitivity of the element is R(V/W), then R is ##EQU3## From the above, it is found that the sensitivity R is inversely proportional to the carrier concentration n of the compound semiconductor 2 and is proportional to the element resistance r.
As described above, in the prior art infrared detector, in order to increase the sensitivity R, it is necessary to increase the resistance r to make the carrier concentration n quite small and to make the layer thickness t of the compound semiconductor 2 quite thin. The value of n has been lowered almost to the limit by advancing crystal growth techniques. However, the layer thickness t of the compound semiconductor layer 2 is required to be larger than some minimum value in order to effectively absorb the incident infrared rays. For example, in a case where infrared rays have wavelengths of 8 to 14 microns, it is required that the thickness t be larger than 15 to 20 microns, making it impossible to reduce the layer thickness t of the compound semiconductor 2. Thus, the layer thickness t is determined by conditions of such as the wavelength of the infrared rays to be absorbed and the element resistance is determined in accordance therewith, and the sensitivity is also restricted thereby.